The present invention relates to optical and electromagnetic sensor systems, and more particularly to such systems which employ spherical or hemispherical Luneburg lens.
Luneburg lens were first proposed by R. K. Luneburg in the 1940's, and are discussed in his textbook "Mathematical Theory of Optics", R. K. Luneburg, University of California Press, Berkeley and Los Angeles, 1964, Library of Congress Catalog #64-19010, pages 187-188. The Luneburg lens may be described simply as a sphere whose index of refraction is graded radially from its center to the outer surface according to the relationship n(r)=(2-r.sup.2).sup.1/2. The Luneburg lens has the property that plane waves from a distant source are imaged at the end of the diameter of the lens which is in the direction of the incoming wave. This property is illustrated in FIG. 1.
However, to applicant's knowledge, no light-gathering Luneburg lens has ever been built and used successfully. Poor approximations have been made but the lack of a suitable material (glass) of the required index of refraction prevented realization. Some radar antennas, which provide hemispherical coverage, have been used for ground based surveillance radar with excellent results; the radial gradation of refractive index was achieved by dielectric loading of a foam low-dielectric constant medium.
It would therefore represent an advance in the art to provide a Luneburg lens useful at optical and millimeter wave frequencies.
The technology of low-refractive index optical materials has been studied, although the focus of applications has been for uses in insulating transparent spacers for house walls and windows, and lately as materials for Cerenkov radiation detectors. One class of such materials is known as aerogels. See, e.g., "Aerogels," J. Fricke, Scientific American, May, 1988, pages 92-97. Rayleigh scattering of light is observed from such materials, which occurs in a medium which contains inhomogeneities smaller than the visible wavelength itself. This indicates that the microscopic structure of these materials consists of primary particles smaller than 1 nanometer. The process of manufacture can produce a very light material whose density is only a few percent of that of ordinary glass by evaporating the liquid used in manufacture and leaving a delicate skeleton structure of silicon.
The properties of aerogel are quite remarkable. Achievable density can be small compared to ordinary solids. Its refractive index, depending on the process of manufacture, has been made as low as 1.02, or can be made larger, for example, .sqroot.2, by manufacturing the aerogel material with increased density. Applied merely as a thermal insulator, it may have one third the conduction of double pane silver coated glass panels containing argon gas, which is thought to be very effective.